The present invention relates to a cellular mobile communication system.
In the cellular mobile communication system, areas in which of mobile terminals are capable of communication are divided into a plurality of cells which neighbor with or partially superpose onto one another. A plurality of mobile terminals located within a cell can make access concurrently to the associated one of base stations which are disposed on a cell-by-cell basis to thereby allow communications between the mobile terminals as well as the mobile terminals and stationary or fixed terminals. In order that a number of mobile terminals within a given cell can make access concurrently to one base station which controls that cell, the technique for avoiding interference among the communication channels plays a very important role in the cellular mobile communication system. Communication systems have been proposed that address the problem of inter-channel interferences such known system and for example:
(1) a frequency division multiple access (FDMA) system,
(2) a time-division multiple access (TDMA) system,
(3) a code-division multiple access (CDMA) system, and
(4) a hybrid system corresponding to a combination of the above-mentioned systems (1) to (3).
According to the FDMA system, a frequency band used for communication with the mobile terminals is divided into a plurality of bands. The base station within a given cell allocates useable bands to a plurality of mobile terminals, respectively, so that communication can be carried out without incurring superposition of spectrums. On the other hand, according to the TDMA scheme, a plurality of mobile terminals can perform communication with the base station in such a manner that signals do not temporally overlap with one another in a same carrier frequency band. Further, in the CDMA system, specific codes are allocated to the individual mobile terminals, respectively. The sender station sends a signal modulated with the codes dispersed therein by using a same carrier frequency, while the receiver station identifies the desired channel by detecting synchronism with the code allocated to that station.
In general, the electromagnetic wave signal emitted in the air undergoes attenuation as the distance from the sender station increases, approximately in proportion to a power exponent of the distance and becomes covered with noise in the area far away from the sender station. Accordingly, in the areas which are sufficiently remote from one another, it is possible to perform communications by using electromagnetic waves of a same frequency independent of one another. This is the principle of cell repetition adopted in the cellular mobile communication system. In this case, the maximum number of the mobile terminals which can make access to one base station concurrently does not depend on the size or area of the cell. Under the circumstances, there is adopted in some cellular mobile communication systems a so-called micro-cellulation scheme for decreasing the radius or radii of the individual cells as one of the methods for coping with the increase in the traffic volume.
Parenthetically, the principle of the cell repetition and the cellular mobile communication system are described in detail in Y. Okumura and M. Shinshi "IDOUTSUUSHIN NO KISO (BASIS OF MOBILE COMMUNICATION)": The Institute of Electronics, Information and Communication Engineers of Japan, pp.188-217, published in May, 1991.
One of the problems encountered in practical application of the cellular mobile communication system is seen in that the traffic volume in each cell will vary significantly depending on the locations or places and time zones. By way of example, a residential district covered by one cell and a commercial district of another cell may remarkably differ from each other in respect to the traffic even when both cells have a same area or size. Further, in a place located around the railway station or a stadium, the traffic volume will become different between a rush-hour zone and the other time zones or between the event-hour zone and other time zones for between a fixture day and other).
However, the cellular mobile communication system known heretofore is so designed that the radii as well as the areas of the cells controlled by the respective base stations are equal to one another without exception, wherein the micro-cellulation (or cell contraction) is effected with reference to a district where the traffic is maximum. As a consequence, there arises unavoidably such situation where the number of cells becomes insufficient or excessive regardless of the actual call demand in dependence on the locations and/or time zones, incurring increase in the cost involved in services and maintenance of basic equipment due to increase in the number of the base stations as well as complication of the system due to registration of locations of the mobile terminals and the liken thus giving rise to problems.